Loading with micro-nanosized α-MnO2 efficiently promotes the removal of arsenite and arsenate by biochar derived from maize straw waste: Dual role of deep oxidation and adsorption
Graphical abstract
Introduction
Arsenic (As), as the priority toxic substance identified by the World Health Organization (WHO), accumulates in the aquatic environment and poses a potential threat to the safety of drinking water. Billions of people worldwide are facing the threat of As (J. Wang et al., 2020; J. Wang et al., 2021; Zhou et al., 2021). Studies have shown that As levels in water in several watersheds around China regularly exceed 10.0 μg/L (WHO safety threshold for drinking water) (Sanjrani et al., 2019) . As usually presents in the form of pentavalent arsenate As (V) and trivalent arsenite As (III) in the groundwater and surface water (Frankenberger, 2001). Long-term exposure to either As (III) or As (V) can result in serious health issues, such as cardiovascular disease and even cancer (Sharma et al., 2014) . Thus far, developing simple but efficient techniques to remove As (III, V) from the aqueous environment become a research priority.
Adsorption, owing to the simplicity and potential for regeneration, has been proven as one of the promising methods to remediate the polluted waters (Han et al., 2020; He et al., 2018; Liang et al., 2019). Biochar (BC, a carbonaceous material pyrolyzed from renewable biomass wastes (e.g., rice straw and animal manure)) has been recognized as a low-cost and eco-friendly adsorbent (Bian et al., 2019; Singh et al., 2021; Zhang et al., 2020). Sekulic et al. (2018) have estimated that the adsorption costs of BC for Pb (II), Cd (II), Ni (II), naproxen, and chlorophenols were about five times less costly than the commercial activated carbons. Meanwhile, BC production facilitates the recycling of agricultural waste which accounts for about 2/3 of the total biomass resources (Lehmann et al., 2015). By the year 2100, the global production of BC will reach 5.5–9.5 gigatons (Gt) per year (Lehmann et al., 2006). To date, many studies have shown that BC exhibited good performance in the removal of cationic heavy metals such as Cd (II) (Han et al., 2017), Cu (II) (He et al., 2021), and Pb (II) (Liang et al., 2017). However, due to the negatively charged surface (Benis et al., 2020), most of the pristine BC had limited abilities for the removal of anions including As (III) and As (V) (Li et al., 2021; Wang et al., 2015). Therefore, pristine BC often requires extra artificial modification to remediate As pollution.
One of the promising modification approaches is to load BC with a material with good adsorption capacity but relatively low production cost. However, the difficulty in screening such a material is that As (III) usually has a much lower sorption affinity than As (V), and thus pre-oxidation of As (III) to As (V) involving oxidizing substances is crucial for deep removal of As (III) (Ge et al., 2017). Among various materials, manganese dioxide (MnO2), which is an important naturally occurring reactants (Luo et al., 2017) and has displayed vast potential in various environmental applications, e.g., improving U (VI) adsorption (B. Wang et al., 2021), promoting desalination (Ma et al., 2019) and enhancing the oxidation of volatile organic compounds (Wu et al., 2020), is one of the good material candidates to load onto BC. In contrast to iron minerals such as nanoscale zero-valent iron which indirectly oxidized As (III) by the intermediates formed (Komárek, 2019; Xu et al., 2021), MnO2 can directly and efficiently oxidize the As (III) and its oxidation is an irreversible process (Cuong et al., 2021). Also, MnO2 can also remove As (V) as a potential adsorbent through the electrostatic retention, ion exchange (Cuong et al., 2021), and surface complexation (Manning et al., 2002). MnO2 has diverse crystalline phases (such as α-, β-, γ-, and δ-forms) depending on synthesis conditions (Mallakpour and Madani, 2016). Compared to other phases of MnO2, α-MnO2 exhibits admirable properties due to easy release of lattice oxygen, high oxidation state (Liu et al., 2009), and abundant hydroxyl groups (Lin et al., 2016). These characteristics endow α-MnO2 with a greater potential to remove As (III, V) (Luo et al., 2017). However, the oxidation and adsorption capacities of bulk α-MnO2 particle are relatively low due to its limited specific surface areas (SSAs). Micro-nanosized (<1000.0 nm) α-MnO2 particle is potentially more favorable for the oxidation and adsorption of As species due to the larger SSA and quantum size effect relative to the bulk one (Con et al., 2013). Nevertheless, micro-nanosized α-MnO2 particles tend to agglomerate in aqueous solutions, which is limited in practical application (Che et al., 2022). Composting with BC happens to help mitigate the self-agglomeration of nanosized metal oxides (Liu et al., 2020; Lyu et al., 2020). Therefore, compositing micro-nanosized α-MnO2 with BC would not only effectively alleviate the agglomeration of micro-nanosized α-MnO2, but also could be expected to more strongly improve the removal of As (III, V) in comparison to BC/bulk α-MnO2 composite reported in the literature (e.g., Cuong et al. (2021) and Y. Wang et al. (2020)). However, it is short of research about the removal efficiency of As (III, V) by BC/micro-nanosized α-MnO2 (BM) composite. During the As (III) removal by the composite, adsorption was inevitably accompanied by oxidation. In this study, the revealing of role of As (III) oxidation and As (III, V) adsorption can provide insightful details about the As (III) removal mechanisms by the BM composites.
Herein, we synthesized BM composites under varying mass ratios of C to Mn considering that the ratio of BC to metal oxide of the composite has been largely demonstrated as one of important factors influencing its removal efficiency for As (III, V) (Cuong et al., 2021), and scrutinized (1) the removal efficiency of BM composites for As (III, V) from aqueous solutions, (2) the reusability of the composites, and (3) the role of As (III) oxidation and As (III, V) adsorption mechanisms in the As (III, V) removal with the use of multiple characterization techniques, e.g., X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) analysis.
Section snippets
Preparation of biochar, micro-nanosized α-MnO2, and their composites
Maize straw (a typical biomass waste) collected from a farmland in Henan Province, China, was used to prepare BC at 450.0 °C. Under 450.0 °C, the biomass was not only well charred (Keiluweit et al., 2010), but also had relatively abundant functional groups (Han et al., 2021) which would provide good basis for MnO2 loading. In brief, the straw was dried and ground to less than 2.0 mm, and was then carbonized in a furnace at 450.0 °C for 2.0 h under N2 protection, with a heating rate of
Successful synthesis of biochar/micro-nanosized α-MnO2 composite
The XRD and FTIR spectra were performed to characterize the crystallinity and functional groups of the composites, respectively. In XRD spectra (Fig. 2a), it was seen that the pattern of the synthesized MnO2 matched quite well with the hollandite-type MnO2 (α-MnO2, JCPDS 44-0141), revealing that α-MnO2 was the main crystallographic phase. Like the pyrolytic carbon reported previously (Keiluweit et al., 2010), the pristine BC sample showed typical diffraction peaks at 2θ = 26.7o, corresponding
Conclusions
This study demonstrated that compositing with micro-nanosized α-MnO2 clearly enhanced the SSA of BC by 7.5–13.5 times, and efficiently overcome the weak As (III, V) removal performance of pristine BC, owing to the reinforced oxidation ability delivered by micro-nanosized α-MnO2 substrate within the composite and the stronger adsorption capacity. Both α-MnO2 and BC substrate within BM composites played roles in the adsorption of As (III, V). The Mn-OH, BC-COOH, and BC-OH functional groups were
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
CRediT authorship contribution statement
Biao Zhang: Investigation, Methodology, Writing – original draft. Lanfang Han: Funding acquisition, Project administration, Formal analysis, Writing – review & editing. Ke Sun: Writing – review & editing. Chuanxin Ma: Writing – review & editing. Jiehong He: Investigation. Liying Chen: Investigation. Jie Jin: Writing – review & editing. Fangbai Li: Supervision, Writing – review & editing. Zhifeng Yang: Conceptualization, Funding acquisition, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the Program for Guangdong Introducing Innovative and Entrepreneurial Teams (2019ZT08L213), National Natural Science Foundation of China (42007013), and Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0403).
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